Universal Mobile Telecommunication System (UMTS) system, which is based on Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS) of European mobile communication systems, is a third generation asynchronous mobile communication system adopting wideband Code Division Multiple Access (CDMA). The 3rd Generation Partnership Project (3GPP) developing the UMTS standardization is discussing Long Term Evolution (LTE) system as the next-generation mobile communication system of the UMTS system. Aiming at commercialization in about 2010, the LTE plans to support the high-speed packet based communication at the transmission rate of 100 Mbps at maximum. For the LTE system, a method for decreasing the number of nodes in the communication channel by simplifying the network configuration or a method for approaching the wireless protocol to the radio channel at maximum is under discussion.
As such, the communication systems are evolving to service the higher speed data or addressing issues in the implementation by modifying the standard. In this evolution, various systems can coexist in the same region according to compatibility level with the existing systems. For example, a new system (such as the LTE system) evolved from the existing system can be installed in a region where the legacy system such as GSM/GPRS/Enhanced Data rates for GSM Evolution (EDGE) (also referred to as an EDGE system) or UMTS/High Speed Packet Access (HSPA) (also referred to as a HSPA system) resides. Also, in the evolution, multimode terminals supporting various Radio Access Technologies (RATs) are introduced.
When communicating within one system, the multimode terminal measures a signal strength of another system for its seamless handover. In order to do so, the standard defines a GAP for the inter-RAT measurement.
Meanwhile, the multimode terminal drives a chipset, which does not operate for the power saving, in a power saving mode, or turns off the chipset. For example, in the LTE mode, the terminal can turn off the HEDGE baseband and the RF chipset. However, for the inter-RAT measurement, the terminal should turn on the corresponding chipset if it needs to and measure another system. When the chipset is turned off, the previous timing information is not preserved. Therefore, the EDGE system requiring the accurate timing information cannot carry out the measurement by itself. Further, since the chipset in the power-off state cannot acquire the accurate GAP location, the start point for the measurement is unknown.
As discussed above, for the multimode terminal supporting the various RATs, a method for error-free or reliable inter-RAT measurement is sought.